What is "slack-water navigation"

Slack \Slack\, a. [Compar. Slacker; superl. Slackest.] [OE. slak, AS. sleac; akin to OS. slak, OHG. slah, Prov. G. schlack, Icel. slakr, Sw. slak; cf. Skr. s[.r]j to let loose, to throw. Cf. Slake.] Lax; not tense; not hard drawn; not firmly extended; as, a slack rope.

2. Weak; not holding fast; as, a slack hand.
--Milton.

3. Remiss; backward; not using due diligence or care; not earnest or eager; as, slack in duty or service.

The Lord is not slack concerning his promise, as some men count slackness.
--2 Pet. iii. 9.

4. Not violent, rapid, or pressing; slow; moderate; easy; as, business is slack. ``With slack pace.''
--Chaucer.

C?sar . . . about sunset, hoisting sail with a slack southwest, at midnight was becalmed.
--Milton.

Slack in stays (Naut.), slow in going about, as a ship.

Slack water, the time when the tide runs slowly, or the water is at rest; or the interval between the flux and reflux of the tide.

Slack-water navigation, navigation in a stream the depth of which has been increased, and the current diminished, by a dam or dams.

Syn: Loose; relaxed; weak; remiss; backward; abated; diminished; inactive; slow; tardy; dull.